PHYSICSPART – II

TEXTBOOK FOR CLASS XII

2020-21

2020-21

PHYSICSPART – II

TEXTBOOK FOR CLASS XII

2020-21

First EditionMarch 2007 Chaitra 1928

ReprintedDecember 2007 Agrahayana 1929

December 2008 Pausha 1930

December 2009 Pausha 1931

January 2011 Pausha 1932

January 2012 Magha 1933

December 2012 Agrahayana 1934

November 2013 Kartika 1935

December 2014 Pausha 1936

December 2015 Agrahayana 1937

February 2017 Magha 1938

December 2017 Pausha 1939

February 2018 Magha 1939

February 2019 Magha 1940

October 2019 Ashwina 1941

PD 450T BS

© National Council of EducationalResearch and Training, 2007

`140.00

Printed on 80 GSM paper with NCERT

watermark

Published at the Publication Division by theSecretary, National Council of Educational

Research and Training, Sri Aurobindo

Marg, New Delhi 110 016 and printed at India

Offset Printers Pvt. Ltd., B-20/1, Okhla

Industrial Area, Phase-II, New Delhi -110 020

ISBN 81-7450-631-4 (Part I)

ISBN 81-7450-671-3 (Part II)

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FOREWORD

The National Curriculum Framework (NCF), 2005 recommends that children’s life at school mustbe linked to their life outside the school. This principle marks a departure from the legacy of bookishlearning which continues to shape our system and causes a gap between the school, home andcommunity. The syllabi and textbooks developed on the basis of NCF signify an attempt to implementthis basic idea. They also attempt to discourage rote learning and the maintenance of sharpboundaries between different subject areas. We hope these measures will take us significantlyfurther in the direction of a child-centred system of education outlined in the National Policy onEducation (NPE), 1986.

The success of this effort depends on the steps that school principals and teachers will take toencourage children to reflect on their own learning and to pursue imaginative activities and questions.We must recognise that, given space, time and freedom, children generate new knowledge by engagingwith the information passed on to them by adults. Treating the prescribed textbook as the sole basisof examination is one of the key reasons why other resources and sites of learning are ignored.Inculcating creativity and initiative is possible if we perceive and treat children as participants inlearning, not as receivers of a fixed body of knowledge.

These aims imply considerable change in school routines and mode of functioning. Flexibility inthe daily time-table is as necessary as rigour in implementing the annual calendar so that therequired number of teaching days are actually devoted to teaching. The methods used for teachingand evaluation will also determine how effective this textbook proves for making children’s life atschool a happy experience, rather than a source of stress or boredom. Syllabus designers have triedto address the problem of curricular burden by restructuring and reorienting knowledge at differentstages with greater consideration for child psychology and the time available for teaching. The textbookattempts to enhance this endeavour by giving higher priority and space to opportunities forcontemplation and wondering, discussion in small groups, and activities requiring hands-onexperience.

The National Council of Educational Research and Training (NCERT) appreciates the hardwork done by the textbook development committee responsible for this book. We wish to thank theChairperson of the advisory group in science and mathematics, Professor J.V. Narlikar and theChief Advisor for this book, Professor A.W. Joshi for guiding the work of this committee. Severalteachers contributed to the development of this textbook; we are grateful to their principals formaking this possible. We are indebted to the institutions and organisations which have generouslypermitted us to draw upon their resources, material and personnel. We are especially grateful tothe members of the National Monitoring Committee, appointed by the Department of Secondaryand Higher Education, Ministry of Human Resource Development under the Chairpersonship ofProfessor Mrinal Miri and Professor G.P. Deshpande, for their valuable time and contribution. Asan organisation committed to systemic reform and continuous improvement in the quality of itsproducts, NCERT welcomes comments and suggestions which will enable us to undertake furtherrevision and refinement.

Director

New Delhi National Council of Educational20 November 2006 Research and Training

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PREFACE

It gives me pleasure to place this book in the hands of the students, teachers and thepublic at large (whose role cannot be overlooked). It is a natural sequel to the Class XI

textbook which was brought out in 2006. This book is also a trimmed version of thetextbooks which existed so far. The chapter on thermal and chemical effects of currenthas been cut out. This topic has also been dropped from the CBSE syllabus. Similarly,

the chapter on communications has been substantially curtailed. It has been rewrittenin an easily comprehensible form.

Although most other chapters have been based on the earlier versions, several parts

and sections in them have been rewritten. The Development Team has been guided bythe feedback received from innumerable teachers across the country.

In producing these books, Class XI as well as Class XII, there has been a basic

change of emphasis. Both the books present physics to students without assumingthat they would pursue this subject beyond the higher secondary level. This new viewhas been prompted by the various observations and suggestions made in the National

Curriculum Framework (NCF), 2005. Similarly, in today’s educational scenario wherestudents can opt for various combinations of subjects, we cannot assume that a physicsstudent is also studying mathematics. Therefore, physics has to be presented, so to

say, in a stand-alone form.As in Class XI textbook, some interesting box items have been inserted in many

chapters. They are not meant for teaching or examinations. Their purpose is to catch

the attention of the reader, to show some applications in daily life or in other areas ofscience and technology, to suggest a simple experiment, to show connection of conceptsin different areas of physics, and in general, to break the monotony and enliven the

book.Features like Summary, Points to Ponder, Exercises and Additional Exercises at

the end of each chapter, and Examples have been retained. Several concept-basedExercises have been transferred from end-of-chapter Exercises to Examples withSolutions in the text. It is hoped that this will make the concepts discussed in thechapter more comprehensible. Several new examples and exercises have been added.Students wishing to pursue physics further would find Points to Ponder and AdditionalExercises very useful and thoughtful. To provide resources beyond the textbook andto encourage eLearning, each chapter has been provided with some relevant websiteaddresses under the title ePhysics. These sites provide additional materials on specifictopics and also provide learners the opportunites for interactive demonstrations/experiments.

The intricate concepts of physics must be understood, comprehended andappreciated. Students must learn to ask questions like ‘why’, ‘how’, ‘how do we knowit’. They will find almost always that the question ‘why’ has no answer within the domainof physics and science in general. But that itself is a learning experience, is it not? Onthe other hand, the question ‘how’ has been reasonably well answered by physicists inthe case of most natural phenomena. In fact, with the understanding of how thingshappen, it has been possible to make use of many phenomena to create technologicalapplications for the use of humans.

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For example, consider statements in a book, like ‘A negatively charged electron isattracted by the positively charged plate’, or ‘In this experiment, light (or electron)behaves like a wave’. You will realise that it is not possible to answer ‘why’. This questionbelongs to the domain of philosophy or metaphysics. But we can answer ‘how’, we canfind the force acting, we can find the wavelength of the photon (or electron), we can

determine how things behave under different conditions, and we can develop instrumentswhich will use these phenomena to our advantage.

It has been a pleasure to work for these books at the higher secondary level, along

with a team of members. The Textbook Development Team, the Review Team and EditingTeams involved college and university teachers, teachers from Indian Institutes ofTechnology, scientists from national institutes and laboratories, as well as higher

secondary teachers. The feedback and critical look provided by higher secondaryteachers in the various teams are highly laudable. Most box items were generated bymembers of one or the other team, but three of them were generated by friends and

well-wishers not part of any team. We are thankful to Dr P.N. Sen of Pune, ProfessorRoopmanjari Ghosh of Delhi and Dr Rajesh B Khaparde of Mumbai for allowing us touse their box items, respectively in Chapters 3, 4 (Part I) and 9 (Part II). We are very

thankful to the members of the Review and Editing Workshops to discuss and refinethe first draft of the textbook. We also express our gratitude to Prof. Krishna Kumar,Director, NCERT, for entrusting us with the task of presenting this textbook as a part of

the national effort for improving science education. I also thank Prof. G. Ravindra, JointDirector, NCERT, for his help from time-to-time. Prof. Hukum Singh, Head, Departmentof Education in Science and Mathematics, NCERT, was always willing to help us in our

endeavour in every possible way.We welcome suggestions and comments from our valued users, especially students

and teachers. We wish our young readers a happy journey into the exciting realm of

physics.

A. W. JOSHI

Chief Advisor

Textbook Development Committee

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TEXTBOOK DEVELOPMENT COMMITTEE

CHAIRPERSON, ADVISORY COMMITTEE FOR TEXTBOOKS IN SCIENCE AND MATHEMATICS

J.V. Narlikar, Emeritus Professor, Inter-University Centre for Astronomy and Astrophysics(IUCAA), Ganeshkhind, Pune University Campus, Pune

CHIEF ADVISOR

A.W. Joshi, Honorary Visiting Scientist, National Centre for Radio Astrophysics (NCRA), PuneUniversity Campus, Pune (Formerly Professor at Department of Physics, University of Pune)

MEMBERS

A.K. Ghatak, Emeritus Professor, Department of Physics, Indian Institute of Technology,New Delhi

Alika Khare, Professor, Department of Physics, Indian Institute of Technology, Guwahati

Anjali Kshirsagar, Reader, Department of Physics, University of Pune, Pune

Anuradha Mathur, PGT , Modern School, Vasant Vihar, New Delhi

Atul Mody, Lecturer (S.G.), VES College of Arts, Science and Commerce, Mumbai

B.K. Sharma, Professor, DESM, NCERT, New Delhi

Chitra Goel, PGT, Rajkiya Pratibha Vikas Vidyalaya, Tyagraj Nagar, New Delhi

Gagan Gupta, Reader, DESM, NCERT, New Delhi

H.C. Pradhan, Professor, Homi Bhabha Centre of Science Education (TIFR), Mumbai

N. Panchapakesan, Professor (Retd.), Department of Physics and Astrophysics, University ofDelhi, Delhi

R. Joshi, Lecturer (S.G.), DESM, NCERT, New Delhi

S.K. Dash, Reader, DESM, NCERT, New Delhi

S. Rai Choudhary, Professor, Department of Physics and Astrophysics, University of Delhi, Delhi

S.K. Upadhyay, PGT, Jawahar Navodaya Vidyalaya, Muzaffar Nagar

S.N. Prabhakara, PGT, DM School, Regional Institute of Education (NCERT), Mysore

V.H. Raybagkar, Reader, Nowrosjee Wadia College, Pune

Vishwajeet Kulkarni, Teacher (Grade I ), Higher Secondary Section, Smt. Parvatibai ChowguleCollege, Margao, Goa

MEMBER-COORDINATOR

V.P. Srivastava, Reader, DESM, NCERT, New Delhi

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ACKNOWLEDGEMENTS

The National Council of Educational Research and Training acknowledges the valuablecontribution of the individuals and organisations involved in the development of Physics Textbookfor Class XII. The Council also acknowledges the valuable contribution of the following academicsfor reviewing and refining the manuscripts of this book:

Anu Venugopalan, Lecturer, School of Basic and Applied Sciences, GGSIP University, Delhi; A.K.Das, PGT, St. Xavier’s Senior Secondary School, Delhi; Bharati Kukkal, PGT, Kendriya Vidyalaya,Pushp Vihar, New Delhi; D.A. Desai, Lecturer (Retd.), Ruparel College, Mumbai; Devendra Kumar,PGT, Rajkiya Pratibha Vikas Vidyalaya, Yamuna Vihar, Delhi; I.K. Gogia, PGT, Kendriya Vidyalaya,Gole Market, New Delhi; K.C. Sharma, Reader, Regional Institute of Education (NCERT), Ajmer;M.K. Nandy, Associate Professor, Department of Physics, Indian Institute of Technology, Guwahati;M.N. Bapat, Reader, Regional Institute of Education (NCERT), Mysore; R. Bhattacharjee, Asstt.Professor, Department of Electronics and Communication Engineering, Indian Institute of Technology,Guwahati; R.S. Das, Vice-Principal (Retd.), Balwant Ray Mehta Senior Secondary School, LajpatNagar, New Delhi; Sangeeta D. Gadre, Reader, Kirori Mal College, Delhi; Suresh Kumar, PGT, DelhiPublic School, Dwarka, New Delhi; Sushma Jaireth, Reader, Department of Women’s Studies, NCERT,New Delhi; Shyama Rath, Reader, Department of Physics and Astrophysics, University of Delhi,Delhi; Yashu Kumar, PGT, Kulachi Hans Raj Model School, Ashok Vihar, Delhi.

The Council also gratefully acknowledges the valuable contribution of the following academicsfor the editing and finalisation of this book: B.B. Tripathi, Professor (Retd.), Department of Physics,Indian Institute of Technology, New Delhi; Dipan K. Ghosh, Professor, Department of Physics,Indian Institute of Technology, Mumbai; Dipanjan Mitra, Scientist, National Centre for RadioAstrophysics (TIFR), Pune; G.K. Mehta, Raja Ramanna Fellow, Inter-University Accelerator Centre,New Delhi; G.S. Visweswaran, Professor, Department of Electrical Engineering, Indian Institute ofTechnology, New Delhi; H.C. Kandpal, Head, Optical Radiation Standards, National PhysicalLaboratory, New Delhi; H.S. Mani, Raja Ramanna Fellow, Institute of Mathematical Sciences,Chennai; K. Thyagarajan, Professor, Department of Physics, Indian Institute of Technology, NewDelhi; P.C. Vinod Kumar, Professor, Department of Physics, Sardar Patel University, VallabhVidyanagar, Gujarat; S. Annapoorni, Professor, Department of Physics and Astrophysics, Universityof Delhi, Delhi; S.C. Dutta Roy, Emeritus Professor, Department of Electrical Engineering, IndianInstitute of Technology, New Delhi; S.D. Joglekar, Professor, Department of Physics, Indian Instituteof Technology, Kanpur; V. Sundara Raja, Professor, Sri Venkateswara University, Tirupati.

The Council also acknowledges the valuable contributions of the following academics forrefining the text in 2017: A.K. Srivastava, Assistant Professor, DESM, NCERT, New Delhi; ArnabSen, Assistant Professor, NERIE, Shillong; L.S. Chauhan, Assistant Professor, RIE, Bhopal;O.N. Awasthi, Professor (Retd.), RIE, Bhopal; Rachna Garg, Professor, DESM, NCERT, NewDelhi; Raman Namboodiri, Assistant Professor, RIE, Mysuru; R.R. Koireng, Assistant Professor,DCS, NCERT, New Delhi; Shashi Prabha, Professor, DESM, NCERT, New Delhi; and S.V. Sharma,Professor, RIE, Ajmer.

Special thanks are due to Hukum Singh, Professor and Head, DESM, NCERT for his support.The Council also acknowledges the support provided by the APC office and the administrative

staff of the DESM; Deepak Kapoor, Incharge, Computer Station; Inder Kumar, DTP Operator;Mohd. Qamar Tabrez and Hari Darshan Lodhi Copy Editor ; Rishi Pal Singh, Sr. Proof Reader,

NCERT and Ashima Srivastava, Proof Reader in shaping this book.The contributions of the Publication Department in bringing out this book are also duly

acknowledged.

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Contents of Physics Part I

Class XII

CHAPTER ONE

ELECTRIC CHARGES AND FIELDS 1

CHAPTER TWO

ELECTROSTATIC POTENTIAL AND CAPACITANCE 51

CHAPTER THREE

CURRENT ELECTRICITY 93

CHAPTER FOUR

MOVING CHARGES AND MAGNETISM 132

CHAPTER FIVE

MAGNETISM AND MATTER 173

CHAPTER SIX

ELECTROMAGNETIC INDUCTION 204

CHAPTER SEVEN

ALTERNATING CURRENT 233

CHAPTER EIGHT

ELECTROMAGNETIC WAVES 269

ANSWERS 288

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CONTENTS

FOREWORD v

PREFACE vii

CHAPTER NINE

RAY OPTICS AND OPTICAL INSTRUMENTS

9.1 Introduction 309

9.2 Reflection of Light by Spherical Mirrors 310

9.3 Refraction 316

9.4 Total Internal Reflection 319

9.5 Refraction at Spherical Surfaces and by Lenses 323

9.6 Refraction through a Prism 330

9.7 Some Natural Phenomena due to Sunlight 332

9.8 Optical Instruments 335

CHAPTER TEN

WAVE OPTICS

10.1 Introduction 351

10.2 Huygens Principle 353

10.3 Refraction and Reflection of Plane Waves using Huygens Principle 355

10.4 Coherent and Incoherent Addition of Waves 360

10.5 Interference of Light Waves and Young’s Experiment 362

10.6 Diffraction 367

10.7 Polarisation 376

CHAPTER ELEVEN

DUAL NATURE OF RADIATION AND MATTER

11.1 Introduction 386

11.2 Electron Emission 387

11.3 Photoelectric Effect 388

11.4 Experimental Study of Photoelectric Effect 389

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11.5 Photoelectric Effect and Wave Theory of Light 393

11.6 Einstein’s Photoelectric Equation: Energy Quantum of Radiation 393

11.7 Particle Nature of Light: The Photon 395

11.8 Wave Nature of Matter 398

11.9 Davisson and Germer Experiment 403

CHAPTER TWELVE

ATOMS

12.1 Introduction 414

12.2 Alpha-particle Scattering and Rutherford’s Nuclear Model of Atom 415

12.3 Atomic Spectra 420

12.4 Bohr Model of the Hydrogen Atom 422

12.5 The Line Spectra of the Hydrogen Atom 428

12.6 DE Broglie’s Explanation of Bohr’s Second Postulate of Quantisation 430

CHAPTER THIRTEEN

NUCLEI

13.1 Introduction 438

13.2 Atomic Masses and Composition of Nucleus 438

13.3 Size of the Nucleus 441

13.4 Mass-Energy and Nuclear Binding Energy 442

13.5 Nuclear Force 445

13.6 Radioactivity 446

13.7 Nuclear Energy 451

CHAPTER FOURTEEN

SEMICONDUCTOR ELECTRONICS: MATERIALS, DEVICES AND SIMPLE CIRCUITS

14.1 Introduction 467

14.2 Classification of Metals, Conductors and Semiconductors 468

14.3 Intrinsic Semiconductor 472

14.4 Extrinsic Semiconductor 474

14.5 p-n Junction 478

14.6 Semiconductor Diode 479

14.7 Application of Junction Diode as a Rectifier 483

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xv

14.8 Special Purpose p-n Junction Diodes 485

14.9 Digital Electronics and Logic Gates 490

APPENDICES 500

ANSWERS 502

BIBLIOGRAPHY 518

INDEX 520

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BACK COVER

(Adapted from http://www.iter.org and

http://www.dae.gov.in)

Cut away view of International Thermonuclear Experimental Reactor (ITER)

device. The man in the bottom shows the scale.

ITER is a joint international research and development project that

aims to demonstrate the scientific and technical feasibility of fusion power.

India is one of the seven full partners in the project, the others beingthe European Union (represented by EURATOM), Japan, the People’s

Republic of China, the Republic of Korea, the Russian Federation and the

USA. ITER will be constructed in Europe, at Cadarache in the South of

France and will provide 500 MW of fusion power.

Fusion is the energy source of the sun and the stars. On earth, fusion

research is aimed at demonstrating that this energy source can be used toproduce electricity in a safe and environmentally benign way, with

abundant fuel resources, to meet the needs of a growing world population.

For details of India’s role, see Nuclear India, Vol. 39, No. 11-12/

May-June 2006, issue available at Department of Atomic Energy (DAE)

website mentioned above.

COVER DESIGN

(Adapted from http://nobelprize.org and

the Nobel Prize in Physics 2006)

Different stages in the evolution of

the universe.

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